In the 1940’s, a reinforced concrete (RC) railway bridge located in central Johannesburg was constructed using Witwatersrand Quartzite, at the time, unknown to be an alkali-silica reactive aggregate. The bridge currently displays signs of distress in the form of severe map-cracking. This study presents preliminary results of a series of tests that have been conducted to characterise the severity of alkali silica reaction (ASR) deterioration in the bridge. Twelve concrete cores were extracted for testing from various locations of the railway bridge, which displayed varying degrees of distress. A visual assessment was conducted globally on the entire structure as well as locally (using modified damage rating index (DRI) approach) on the extracted cores. The findings showed that there was extensive map-cracking present on all thirteen railway bridge elements examined, revealing signs of distress in the bridge. Even elements that were sheltered from exposure to direct rain, such as the underside of three arch surfaces, also exhibited extensive map-cracking. Furthermore, the modified DRI method was a useful technique to assess and compare the relative extent of damage of concrete taken from various locations of a RC structure, using local images of the cylindrical surface of the concrete cores.
Graphical Abstract
The existing cement and concrete standards are not capable of making full use of the current technology capacity due to strong focus on conventional concrete and thus they are not fit for the current and future challenges of construction industry. The paper highlights shortcomings with regard to the implementation of the existing standards. It can be seen that future-oriented standards are generally required to contribute to a lower-carbon footprint of the industry. These changes are significantly more relevant in sub-Saharan Africa, due to the rapidly increasing urbanisation challenge and the enormous potentials to develop lower-carbon technologies than elsewhere in the world.
Graphical abstract
The South African construction industry is currently faced with mounting construction waste and overflowing landfills. Consequently, the feasibility of using of clay masonry rubble brick (CMRB) as a partial replacement for natural coarse aggregate in concrete was investigated. Three concrete mixes were made using a water-to-binder (w/b) ratios of 0.60. A control mix as well as a mix containing 50% and 100 % replacement of coarse Andesite aggregate with CMRB as coarse aggregate was tested. The compressive strength, shrinkage and durability properties (with respect to oxygen permeability, water sorptivity and chloride conductivity) were assessed. The results show that the incorporation 100% MCR brick as coarse aggregate produced the lowest compressive strength result, the highest shrinkage rate and the least resistance to gaseous ingress and chloride conductivity. The concrete mix containing 50% MCR brick exhibited the least resistance to water absorption through capillary action. The mechanical and durability properties of both concrete mix containing MCR brick was largely affected by the porous nature of coarse masonry rubble. The results can be attributed to a relatively high void ratio leading to the presence of more interlinked pathways within the CMRB aggregate.
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